1. Technical Field
The present invention generally relates to a semiconductor memory device, and more particularly, to a semiconductor memory device capable of measuring a write current and a method for measuring a write current.
2. Related Art
In general, a semiconductor memory device is classified into a volatile memory device and a nonvolatile memory device. The volatile memory device loses data stored therein when power supply is cut off, but the nonvolatile memory device maintains data stored therein even though power supply is cut off. The nonvolatile memory device may include various types of memory cells.
The nonvolatile memory device may be divided into a flash memory device, a ferroelectric RAM (FRAM) using a ferroelectric capacitor, a magnetic RAM (MRAM) using a tunneling magneto-resistive (TMR) layer, a phase change memory device using chalcogenide alloys and the like, depending on the structure of memory cells. In particular, the phase change memory device is a nonvolatile memory device using phase change based on temperature change, that is, resistance change. For this reason, the phase change memory device is also referred to as a variable resistance memory device.
The phase change memory device includes memory cells formed of a phase change material, for example, a chalcogenide alloy which is a germanium(Ge)-antimony(Sb)-tellurium(Te) compound (GST) (hereafter, referred to as a GST material). The GST material has an amorphous state having a relatively high resistivity and a crystalline state having a relatively low resistivity. The GST material may be heated to program data corresponding to the amorphous state or the crystalline state into the memory cell of the phase change memory device. The amorphous state or the crystalline state of the GST material may be controlled by adjusting the magnitude of a current for heating the GST material and the application time of the current, for example.
As described above, the state of the memory cell of the phase change memory device is changed by an applied write current. Furthermore, the state of the memory cell of the phase change memory device is determined according to how much current the memory cell can pass. When the write current is varied by an effect of a write driver, a peripheral circuit, or another memory cell programmed at the same time during a write operation of the phase change memory device, an unexpected resistance distribution of the memory cell may be formed. For this reason, a test for the memory cell cannot be accurately performed.
Therefore, there is a demand for a method capable of measuring a write current applied to a memory while excluding an effect of a peripheral circuit or another memory cell programmed at the same time in a test operation of the phase change memory device.